Air flow through a porous refractory was experimentally studied at a large mean free path region and the resulting pressure drop across the porous medium was measured to evaluate the three-dimensional complexity of the refractory. A new method to evaluate the pore structure of a porous medium has been proposed. It is composed of the measurement of pressure drop and the determination of the effective specific surface area of the medium at the same mass flow rate but at a different absolute gas pressure and the evaluation of the pore structure through fractal analysis. This approach is can be used to measure any pore structure.